Transmit/receive systems for imaging devices

ABSTRACT

A transceiver for an ultrasonic imaging device includes a transmit circuit and a receive circuit. The transmit circuit outputs test pulses to a probe including a transducer to generate an image of a test object. A composite signal including the test pulses and a reflected signal is output by the transducer. The receive circuit receives the composite signal including the test pulses and the reflected signal and includes a filter circuit. The filter circuit filters the test pulses from the composite signal and passes the reflected signal. An impedance of the filter circuit is equal to substantially zero when the reflected signal is within a predetermined frequency range.

FIELD

The present disclosure relates to imaging devices and more particularlyto Transmit/Receive (T/R) circuits for imaging devices.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

External features of an object can be viewed by a human eye and capturedvia conventional imaging devices, such as a camera. Internal features ofthe object, however, generally cannot be readily observed. Ultrasonicimaging devices are used in various fields. For example only, anultrasonic imaging device may be used for medical imaging,non-destructive testing, non- and minimally-invasive testing, and otherfields.

An ultrasonic imaging device generally includes one or more transmittersand one or more receivers. The transmitters generate test signals thatare applied to a probe. The probe includes transducers that move basedon the test signals. When the probe is in contact with an object,movement of the transducers causes a wave to propagate through theobject. Reflected waves also cause the transducers to move, and thetransducers output reflected signals. An image of the internal featuresof the object can be generated based on the reflected signals.

SUMMARY

A transceiver for an ultrasonic imaging device includes a transmitcircuit and a receive circuit. The transmit circuit outputs test pulsesto a probe including a transducer to generate an image of a test object.A composite signal including the test pulses and a reflected signal isoutput by the transducer. The receive circuit receives the compositesignal including the test pulses and the reflected signal and includes afilter circuit. The filter circuit filters the test pulses from thecomposite signal and passes the reflected signal. An impedance of thefilter circuit is equal to substantially zero when the reflected signalis within a predetermined frequency range.

In further features, a transceiver for an ultrasonic imaging deviceincludes a transmit circuit and a diode bridge. The transmit circuitoutputs test pulses to a probe including a transducer to generate animage of a test object. A composite signal including the test pulses anda reflected signal is output by the transducer. The diode bridgereceives the composite signal including the test pulses and thereflected signal. The diode bridge filters the test pulses from thecomposite signal and passes the reflected signal. Each of the diodes ofthe diode bridge has a forward transit time that is greater than onedivided by a product of 2π and a minimum value of a predeterminedfrequency range of the reflected signal.

In still further features, a transceiver for an ultrasonic imagingdevice includes a transmit circuit, a diode bridge, and a bias resistor.The transmit circuit outputs test pulses of a first magnitude to a probeincluding a transducer to generate an image of a test object. Acomposite signal including the test pulses and a reflected signal isoutput by the transducer. A maximum magnitude of the reflected signal isless than the first magnitude. The diode bridge receives the compositesignal including the test pulses and the reflected signal and includesdiodes having forward transit times that are each greater than onedivided by a product of 2π and a minimum value of a predeterminedfrequency range of the reflected signal. The bias resistor is connectedbetween a bias voltage and the diode bridge. A second magnitude of thebias voltage is greater than the maximum magnitude and less than thefirst magnitude. The diode bridge filters voltages of the compositesignal that are greater than the bias voltage and passes voltages of thecomposite signal that are less than the bias voltage.

Further areas of applicability of the present disclosure will becomeapparent from the detailed description provided hereinafter. It shouldbe understood that the detailed description and specific examples areintended for purposes of illustration only and are not intended to limitthe scope of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 is a functional block diagram of an example imaging systemaccording to the present disclosure;

FIG. 2 is a schematic of an example transmit/receive path of the imagingsystem according to the present disclosure;

FIG. 3 is a partial schematic of a receive circuit according to thepresent disclosure;

FIG. 4 is a schematic of a small-signal equivalent circuit of thereceive circuit of FIG. 3;

FIG. 5 is an example graph of impedance as a function of frequency; and

FIGS. 6-7 are example graphs of gain as a function of frequency.

DETAILED DESCRIPTION

The following description is merely illustrative in nature and is in noway intended to limit the disclosure, its application, or uses. Forpurposes of clarity, the same reference numbers will be used in thedrawings to identify similar elements. As used herein, the phrase atleast one of A, B, and C should be construed to mean a logical (A or Bor C), using a non-exclusive logical or. It should be understood thatsteps within a method may be executed in different order withoutaltering the principles of the present disclosure.

As used herein, the term module may refer to, be part of, or include anApplication Specific Integrated Circuit (ASIC); an electronic circuit; acombinational logic circuit; a field programmable gate array (FPGA); aprocessor (shared, dedicated, or group) that executes code; othersuitable components that provide the described functionality; or acombination of some or all of the above, such as in a system-on-chip.The term module may include memory (shared, dedicated, or group) thatstores code executed by the processor.

The term code, as used above, may include software, firmware, and/ormicrocode, and may refer to programs, routines, functions, classes,and/or objects. The term shared, as used above, means that some or allcode from multiple modules may be executed using a single (shared)processor. In addition, some or all code from multiple modules may bestored by a single (shared) memory. The term group, as used above, meansthat some or all code from a single module may be executed using a groupof processors. In addition, some or all code from a single module may bestored using a group of memories.

The apparatuses and methods described herein may be implemented by oneor more computer programs executed by one or more processors. Thecomputer programs include processor-executable instructions that arestored on a non-transitory tangible computer readable medium. Thecomputer programs may also include stored data. Non-limiting examples ofthe non-transitory tangible computer readable medium are nonvolatilememory, magnetic storage, and optical storage.

An imaging device, such as an ultrasonic imaging device, includes atransmit circuit and a receive circuit. The transmit circuit selectivelygenerates test pulses. A transducer of a probe moves based on the testpulses to propagate an ultrasonic wave through an object.

The transducer also senses reflected waves. The transducer outputs areflected signal based on reflected waves. The receive circuit receivessignals based on the test pulses and the reflected waves. The receivecircuit filters the test pulses and passes the reflected signal.

The receive circuit may include a diode bridge. The diodes of the diodebridge have forward transit times that are greater than a predeterminedvalue. The predetermined value may be greater than 1/(2*π*f), where f isa predetermined minimum frequency of the reflected signal. Thepredetermined minimum frequency is greater than zero and less than apredetermined maximum frequency of the reflected signal.

Using diodes with forward transit times that are greater than thepredetermined value ensures that a cutoff frequency of the diode bridgeis less than the predetermined minimum frequency. When the cutofffrequency of the diode bridge is less than the predetermined minimumfrequency, the diode bridge has an impedance of substantially zerobetween the predetermined minimum frequency and the predeterminedmaximum frequency. The diode bridge having an impedance of substantiallyzero may mean that the impedance of the diode bridge is approximatelyequal to an equivalent parasitic series resistor. When the forwardtransit times are greater than the predetermined value, a decrease inpower dissipation and/or one or more other benefits may be realizedrelative to a diode bridge with diodes having forward transit times thatare less than the predetermined value.

Referring now to FIG. 1, a functional block diagram of an exampleimplementation of an imaging system 100 is presented. The imaging system100 may be, for example, of a medical imaging device, a non-destructivetesting device, or another suitable type of device. The imaging system100 may be implemented in a portable or a non-portable device. Portabledevices may be powered via one or more batteries, while non-portabledevices may be powered via a utility.

The imaging system 100 includes a transmit/receive (T/R) path 104. Whileonly the T/R path 104 is shown, a given imaging device may include aplurality of T/R paths. A T/R path can also be referred to as a T/Rchannel. The T/R path 104 includes a T/R node 108 that is connected to amultiplexer 112. An imaging probe 116 includes one or more transducers,such as transducer 118, that may be connected to the multiplexer 112 viaone or more electrical connectors 120. In various implementations, themultiplexer 112 may be omitted. For example only, the transducer 118 mayinclude a piezoelectric transducer. In various implementations, morethan one probe may be associated with a given T/R path.

The T/R path 104 includes a control module 124, a T/R circuit 128, anAFE module 136, and an ADC module 144. In various implementations, theT/R circuit 128 and the AFE module 136 may be implemented independentlyor within a single chip. The control module 124 outputs a control signalto the T/R circuit 128 for propagating an ultrasonic wave through anobject.

The T/R circuit 128 includes both a transmit circuit 140 and a receivecircuit 132. The transmit circuit 140 generates a test signal based onthe control signal and outputs the test signal to the T/R node 108. Whenthe transmit circuit 140 is outputting the test signal to the T/R node108, a switch of the multiplexer 112 may be actuated to connect the T/Rnode 108 with the transducer 118. In various implementations, the T/Rnode 108 may be directly connected to the transducer 118. The transducer118 moves based on the test signal and causes a pressure wave topropagate into the object.

Reflected pressure waves also cause the transducer 118 to move. Thetransducer 118 senses reflected waves and outputs a reflected(electrical) signal based on the reflected waves. The T/R node 108receives both the test signal and the reflected signal. The receivecircuit 132 filters/blocks the test signal. The receive circuit 132 mayminimize attenuation of the reflected signal before providing it to theAFE module 136. In various implementations, the receive circuit 132 mayalso perform amplification.

The AFE module 136 may perform one or more analog functions, such asamplifying, and filtering, before outputting an imaging signal to theADC module 144. The ADC module 144 may selectively generate digitalsamples based on the imaging signal and output the digital samples to aprocessing module 150. The processing module 150 may process the digitalsamples output by the ADC module 144 and other ADC modules to generatean image of internal features of the object. The image may be displayedvia a display 154.

Referring now to FIG. 2, a functional block diagram of exampleimplementations of the transmit circuit 140 and the receive circuit 132is presented. A driver 204 may generate the test signal based on thecontrol signal. More specifically, the driver 204 selectively generatesvoltage pulses in the test signal. For example only, the voltage pulsesmay be unipolar pulses between a first predetermined voltage and areference potential (e.g., ground), bipolar pulses between positive andnegative values of a first predetermined voltage, multi-level pulses, oranother suitable type of pulses. For example only, the firstpredetermined voltage may be between 5 Volts (V) and 300 V, inclusive,and may be approximately 100 V in various implementations. By way ofcontrast, the magnitude of the reflected signal may be in the range oftens of microvolts (μV) to hundreds of millivolts (mV) or other voltagesthat are less than the first predetermined voltage.

The operating frequency of the test signal may be between apredetermined minimum frequency and a predetermined maximum frequency.For example only, the predetermined minimum and maximum frequencies maybe approximately 1 megahertz (MHz) and 10 megahertz (MHz), respectively,or other suitable values. The operating frequency of the test signal maybe set based on a user input or another suitable input. The transmitcircuit 140 may output the test signal to the T/R node 108 via a pair ofanti-parallel diodes 208 and 212.

A predetermined frequency range (of interest) of the reflected signalmay be between a predetermined minimum frequency and a predeterminedmaximum frequency. For example only, the predetermined minimum andmaximum frequencies may be approximately 1 megahertz (MHz) and 10megahertz (MHz), respectively, or other suitable values.

The receive circuit 132 may include a filter circuit 214 that mayinclude a diode bridge 216, a clipper circuit 224, and an amplifier 228.A first bias resistor 246 may be connected between a positive biasvoltage 240 and a first node 244 of the diode bridge 216. A second biasresistor 254 may be connected between a negative bias voltage 248 and asecond node 252 of the diode bridge 216. The positive and negative biasvoltages 240 and 248 may be, for example, +/−5 V, +/−12 V, or anothersuitable voltage that is less than the first predetermined voltage andgreater than a maximum value of the reflected signal. The application ofthe bias voltage to the diode bridge 216 causes a bias current to flow.

The diode bridge 216 may include four diodes: a first diode 256, asecond diode 260, a third diode 264, and a fourth diode 268. While thediode bridge 216 is shown and described as including a full-bridge, thediode bridge 216 may include a half-bridge in various implementations.The anodes of the first and third diodes 256 and 264 are connected tothe first node 244. The cathode of the first diode 256 is connected toan input node 272 of the diode bridge 216, and the input node 272 isconnected to the T/R node 108. The cathode of the third diode 264 isconnected to an output node 276 of the diode bridge 216, and the outputnode 276 is connected to the clipper circuit 224 and the amplifier 228.The anodes of the second and fourth diodes 260 and 268 are connected tothe input and output nodes 272 and 276, respectively. The cathodes ofthe second and fourth diodes 260 and 268 are connected to the secondnode 252.

The filter circuit 214 blocks the test signal. The filter circuit 214allows the reflected signal to pass from the input node 272 to theoutput node 276. For example only, because the magnitude of the testsignal is greater than the magnitude of the bias voltage, the diodes ofthe diode bridge 216 are reverse biased when the test signal is presentat the T/R node 108. Accordingly, the diodes of the diode bridge 216prevent current output by the transmit circuit 140 from flowing betweenthe input node 272 and the output node 276. Because the magnitude of thereflected signal is less than the magnitude of the bias voltage,however, the diodes of the diode bridge 216 are forward biased and allowcurrent to flow between the input node 272 and the output node 276.

The clipper circuit 224 may include fifth and sixth diodes 280 and 284.The cathode of the fifth diode 280 and the anode of the sixth diode 284may be connected to the output node 276. The anode of the fifth diode280 and the cathode of the sixth diode 284 may be connected to areference potential, such as ground. The amplifier 228 is also connectedto the output node 276.

The clipper circuit 224 limits the magnitude of the voltage input to theamplifier 228 (i.e., the voltage at the output node 276) to less thanthe magnitude of the bias voltage. The amplifier 228 may include, forexample, a low noise amplifier (LNA) or another suitable type ofamplifier. In various implementations, the clipper circuit 224 and/orthe amplifier 228 may be implemented independently or within the AFEmodule 136.

FIG. 3 is an example schematic of the filter circuit 214 of the receivecircuit 132. FIG. 4 is a small-signal equivalent circuit 400 of thefilter circuit 214 of FIG. 3, assuming that the first and second biasresistors 246 and 254 are greater than the ON resistance (R_(ON)) of thediode bridge 216. Because the diode bridge 216 includes two parallelsignal paths, each path having two diodes in series, the input to outputimpedance of the diode bridge 216 can be approximated in the first-orderas a single equivalent diode for small-signal equivalent circuitpurposes.

Referring now to FIG. 4, the small-signal equivalent circuit 400includes an equivalent (parasitic) series resistor (R_(s)) 404 connectedbetween the input node 272 and a third node 408. The small-signalequivalent circuit 400 also includes an equivalent diode resistor(R_(d)) 412 and a diffusion capacitor (C_(d)) 416 that are bothconnected between the third node 408 and the output node 276. Theresistance of the equivalent diode resistor 412 is generally greaterthan the resistance of the equivalent series resistor 404.

The resistance of the equivalent diode resistor 412 can be determinedbased on:

${R_{d} = \frac{1}{g_{m}}},$

where R_(d) is the resistance of the equivalent diode resistor 412 andg_(m) is the transconductance of the single equivalent diode. Thetransconductance of the single equivalent diode (g_(m)) can bedetermined based on:

${g_{m} = \frac{I_{d}}{V_{t}}},$

where g_(m) is the transconductance of the single equivalent diode,I_(d) is the forward bias current through each diode of the diode bridge216 and is or is approximately one half of the current flowing from thepositive bias voltage 240 to the negative bias voltage 248, and V_(t) isthe thermal voltage of the single equivalent diode. The capacitance ofthe diffusion capacitor 416 can be determined based on:

C _(d) =T _(fw) *g _(m),

where C_(d) is the capacitance of the diffusion capacitor 416, T_(fw) isthe forward transit time (seconds) of the single equivalent diode, andg_(m) is the transconductance of the single equivalent diode.

FIG. 5 is a graph of impedance 504 of the small-signal equivalentcircuit 400 as a function of frequency 508. When the frequency of thereflected signal is less than a cutoff frequency (f_(t)) 512, theimpedance of the small-signal equivalent circuit 400 may beapproximately equal to the sum of resistance of the equivalent dioderesistor 412 and the resistance of the equivalent series resistor 404(i.e., R_(d)+R_(s)). The impedance of the small-signal equivalentcircuit 400 being approximately equal to the sum of the resistance ofthe equivalent diode resistor 412 and the resistance of the equivalentseries resistor 404 may be attributable to the diffusion capacitor 416acting substantially as an open-circuit at frequencies less than thecutoff frequency 512. When acting substantially as an open-circuit maymean that at least a predetermined percent of the current flowingbetween the third node 408 and the output node 276 will be forced toflow through the equivalent diode resistor 412. For example only, thepredetermined percent may be greater than approximately 95 percent, 96percent, 97 percent, 98 percent, 99 percent, or another suitable value.When the frequency of the reflected signal is less than the cutofffrequency 512, the filter circuit 214 will be referred to as operatingin Zone A.

The impedance of the small-signal equivalent circuit 400 may decrease asthe frequency increases between the cutoff frequency 512 and a secondfrequency 516. The decrease in the impedance may be attributable to thediffusion capacitor 416 increasingly acting as a short circuit as thefrequency of the reflected signal increases. At frequencies greater thanthe second frequency 516, the impedance of the small-signal equivalentcircuit 400 may be approximately equal to the resistance of theequivalent (parasitic) series resistor 404 (i.e., R_(s)) only. Thefilter circuit 214 will be referred to as operating in Zone B whenoperating at frequencies that are greater than the second frequency 516.

The impedance of the small-signal equivalent circuit 400 beingapproximately equal to the resistance of the equivalent series resistor404 only may be attributable to the diffusion capacitor 416 effectivelyshunting the equivalent diode resistor 412 during operation in Zone B.More specifically, during operation in Zone B, the diffusion capacitor416 may act substantially as a short circuit. Noise generated by theequivalent diode resistor 412 may therefore be shunted by the diffusioncapacitor 416 and reduce or be a non-contributor to the noise present atthe output node 276 during operation in Zone B.

Acting substantially as a short circuit may mean that the diffusioncapacitor 416 allows at least a first predetermined percent of thecurrent flowing between the third node 408 and the output node 276 tobypass the equivalent diode resistor 412. In this manner, the impedanceassociated with the equivalent diode resistor 412 and the diffusioncapacitor 416 may be substantially zero at frequencies greater than thesecond frequency 516. The impedance associated with the equivalent dioderesistor 412 and the diffusion capacitor 416 being substantially zeromay mean less than a second predetermined percent of the impedanceassociated with the equivalent diode resistor 412 and the diffusioncapacitor 416 impedance at and below the cutoff frequency 512(R_(s)+R_(d)). For example only, this predetermined percentage may beapproximately 5 percent, approximately 4 percent, approximately 3percent, approximately 2 percent, approximately 1 percent, or anothersuitable value. When the impedance associated with the equivalent dioderesistor 412 and the diffusion capacitor 416 is substantially zero, theimpedance of the small-signal equivalent circuit 400 may be said to beequal to substantially zero. The impedance of the small-signalequivalent circuit 400 being substantially zero may mean equal to a sumof the resistance of the equivalent (parasitic) series resistor 404 andthe substantially zero impedance associated with the equivalent dioderesistor 412 and the diffusion capacitor 416.

Cutoff frequency (f_(t)) is related to forward transit time (T_(fw)).For example only, the cutoff frequency can be determined based on:

${f_{t} = {\frac{1}{2*\pi*T_{fw}} = \frac{1}{2*\pi*R_{d}*C_{d}}}},$

where f_(t) is the cutoff frequency, π is the mathematical constant Pi,R_(d) is the resistance of the equivalent diode resistor 412, and R_(s)is the resistance of the equivalent series resistor 404.

The forward transit times of diodes of a given diode bridge may beapproximately 4-6 nanoseconds (ns) in various implementations. However,the given diode bridge would operate in Zone A when the reflected signalis received from the probe 116. If the given diode bridge operates inZone A, the input/output ON resistance of the given diode bridge and thenoise generated by the given diode bridge is inversely related to thebias current.

Accordingly, the bias current would have to be increased in order todecrease the ON resistance of the given diode bridge and the noisegenerated by the given diode bridge. Increasing the bias current,however, increases the amount of power that is dissipated by the givendiode bridge and lost. A decrease in power dissipation may be especiallybeneficial to portable (e.g., handheld) imaging devices that rely on oneor more batteries for power.

An increase in the bias current can be accomplished by decreasing theresistances of the first and second bias resistors used with the givendiode bridge. However, based on the decrease in the load seen by thetransmit circuit 140 and to reduce the noise generated by the biasresistors, external inductors in series with the bias resistors,respectively, may need to be included. The addition of the externalinductors, however, may increase package size and price and may requirethat each T/R circuit be supplied with additional input/output (I/O)pins for external connection to the external inductors.

The first, second, third, and fourth diodes 256, 260, 264, and 268 ofthe present disclosure have forward transit times that are greater thana predetermined period. More specifically, the forward transit times ofthe first, second, third, and fourth diodes 256, 260, 264, and 268 aresuch that the cutoff frequency 512 and the second frequency 516 are lessthan the predetermined minimum frequency of the reflected signal. Forexample only, the forward transit times of the first, second, third, andfourth diodes 256, 260, 264, and 268 can be expressed by:

${T_{fw} > \frac{1}{2*\pi*f_{{Mi}\; n}}},$

where T_(fw) is the forward transit time of the first, second, third,and fourth diodes 256, 260, 264, and 268, π is the mathematical constantPi, and f_(Min) is the predetermined minimum frequency of the reflectedsignal. The predetermined minimum frequency is greater than zero, andthe predetermined maximum frequency is greater than the predeterminedminimum frequency. For example only, if the predetermined minimumfrequency is 1 Megahertz (MHz), the forward transit times may be greaterthan approximately 250 nanoseconds (ns).

When the cutoff frequency and the second frequency are less than thepredetermined minimum frequency, the diode bridge 216 may operate inZone B at operating frequencies between the predetermined minimumfrequency and the predetermined maximum frequency, inclusive. If thediode bridge 216 operates in Zone B, the ON resistance of (and thereforethe insertion loss attributable to) the diode bridge 216 is independentof the bias current.

The diode bridge 216 can therefore operate with a lower bias currentrelative to a diode bridge with diodes having forward transit times thatare less than the predetermined period. For example only, the diodebridge 216 can operate with a bias current of approximately 1.5milliamps (mA) or less while a diode bridge with diodes having forwardtransit times that are less than the predetermined period may operatewith a bias current of approximately 10 mA or more to provide a similartotal input/output ON resistance and a similar amount of noise. Thelower bias current may enable the diode bridge 216 to provide a decreasein the amount of power dissipated relative to the power dissipation of adiode bridge with diodes having forward transit times that are less thanthe predetermined period. For example only, the power dissipation may bereduced by approximately a factor of 10 or more. Additionally, the diodebridge 216 can operate without external inductors, and the diode bridge216 may provide a higher power supply rejection ratio (PSRR) than adiode bridge with diodes having forward transit times that are less thanthe predetermined period.

Referring now to FIG. 6, an example graph of gain 604 of the diodebridge 216 as a function of frequency 608 is presented. The diodes ofthe diode bridge 216 have forward transit times that are greater thanthe predetermined value such that the cutoff frequency and the secondfrequency are less than the predetermined minimum frequency. Exampletrace 612 tracks the gain 604 of the diode bridge 216 as a function ofthe frequency 608 with a bias current of approximately 1.5 milliamps(mA). Example trace 616 tracks the gain 604 of the diode bridge 216 as afunction of the frequency 608 with a bias current of approximately 3.0mA. The similarity of the example traces 612 and 616 may indicate thatthe ON resistance (and therefore the insertion loss) of the diode bridge216 is independent of the bias current when the diodes have forwardtransit times that are greater than the predetermined value.

Referring now to FIG. 7, an example graph of gain 704 as a function offrequency 708 for various diode bridges is presented. Example solidtrace 712 tracks the gain 704 of a second diode bridge as a function ofthe frequency 708. The diodes of the second diode bridge have forwardtransit times that are less than the predetermined value. Example dashedtrace 716 tracks the gain 704 of the diode bridge 216 as a function ofthe frequency 708. The diodes of the diode bridge 216 have forwardtransit times that are greater than the predetermined value. The exampletraces 712 and 716 are graphed with the diode bridge 216 and the seconddiode bridge being biased with a 1.5 mA bias current and driving a 50Ohm load. The difference in the gain 704 between the example traces 712and 716 indicates that the diodes of the diode bridge 216 with forwardtransit times that are greater than the predetermined value may providean improvement in the gain 704 of approximately 6.5 decibels (dB).

The broad teachings of the disclosure Can be implemented in a variety offorms. Therefore, while this disclosure includes particular examples,the true scope of the disclosure should not be so limited since othermodifications will become apparent to the skilled practitioner upon astudy of the drawings, the specification, and the following claims.

1. A transceiver for an ultrasonic imaging device, comprising: atransmit circuit that outputs test pulses to a probe including atransducer to generate an image of a test object, wherein a compositesignal including the test pulses and a reflected signal is output by thetransducer; and a receive circuit that receives the composite signalincluding the test pulses and the reflected signal and that includes afilter circuit that filters the test pulses from the composite signaland passes the reflected signal, wherein an impedance of the filtercircuit is equal to substantially zero when the reflected signal iswithin a predetermined frequency range.
 2. The transceiver of claim 1wherein the filter circuit includes a diode bridge, wherein diodes ofthe diode bridge have forward transit times that are greater than onedivided by a product of 2π and a minimum value of the predeterminedfrequency range.
 3. The transceiver of claim 2 wherein the diode bridgeis a full-bridge.
 4. The transceiver of claim 2 further comprising: abias resistor connected between a bias voltage and the diode bridge,wherein a magnitude of the bias voltage is less than a magnitude of thetest pulses, and wherein a maximum value of the reflected signal is lessthan the magnitude of the bias voltage.
 5. The transceiver of claim 4further comprising a clipper circuit that communicates with an output ofthe filter circuit and that limits a magnitude of the output of thefilter circuit to less than the magnitude of the bias voltage; and anamplifier that amplifies the output of the filter circuit.
 6. Thetransceiver of claim 5 wherein the amplifier includes a low noiseamplifier (LNA).
 7. The transceiver of claim 5 wherein the clippercircuit includes a first diode connected between the output of thefilter circuit and a reference potential and a second diode connectedanti-parallel to the first diode between the output of the filtercircuit and the reference potential.
 8. The transceiver of claim 4wherein zero inductors are connected between the bias voltage and thebias resistor.
 9. The transceiver of claim 1 wherein the filter circuitincludes a bridge of diodes having forward transit times that aregreater than one divided by a product of 2π and a minimum value of thepredetermined frequency range by at least a predetermined amount. 10.The transceiver of claim 1 further comprising: an amplifier thatcommunicates with an output of the filter circuit, that amplifies avoltage at the output, and that outputs an amplified voltage; and aprocessing module that generates a signal for displaying the image ofthe test object based on the amplified voltage.
 11. The transceiver ofclaim 1 wherein the impedance is greater than substantially zero whenthe reflected signal is less than a minimum value of the predeterminedfrequency range.
 12. A transceiver for an ultrasonic imaging device,comprising: a transmit circuit that outputs test pulses to a probeincluding a transducer to generate an image of a test object, wherein acomposite signal including the test pulses and a reflected signal isoutput by the transducer; and a diode bridge that receives the compositesignal including the test pulses and the reflected signal and thatfilters the test pulses from the composite signal and passes thereflected signal, wherein each of the diodes of the diode bridge has aforward transit time that is greater than one divided by a product of 2πand a minimum value of a predetermined frequency range of the reflectedsignal.
 13. The transceiver of claim 12 wherein the forward transit timeis greater than one divided by the product of 2π and the minimum valueof the predetermined frequency range by a predetermined amount.
 14. Thetransceiver of claim 12 wherein the diode bridge is a full-bridge. 15.The transceiver of claim 12 further comprising: a bias resistorconnected between a bias voltage and the diode bridge, wherein amagnitude of the bias voltage is less than a magnitude of the testpulses, and wherein a maximum value of the reflected signal is less thanthe magnitude of the bias voltage.
 16. The transceiver of claim 15further comprising: a clipper circuit that communicates with an outputof the diode bridge and that limits a magnitude of the output of thediode bridge to less than the magnitude of the bias voltage; anamplifier that communicates with an output of the diode bridge, thatamplifies a voltage at the output, and that outputs an amplifiedvoltage; and a processing module that generates a signal for displayingthe image of the test object based on the amplified voltage.
 17. Thetransceiver of claim 16 wherein the amplifier includes a low noiseamplifier (LNA).
 18. The transceiver of claim 15 wherein zero inductorsare connected between the bias voltage and the bias resistor.
 19. Atransceiver for an ultrasonic imaging device, comprising: a transmitcircuit that outputs test pulses of a first magnitude to a probeincluding a transducer to generate an image of a test object, wherein acomposite signal including the test pulses and a reflected signal isoutput by the transducer, and wherein a maximum magnitude of thereflected signal is less than the first magnitude; a diode bridge thatreceives the composite signal including the test pulses and thereflected signal and that includes diodes having forward transit timesthat are each greater than one divided by a product of 2π and a minimumvalue of a predetermined frequency range of the reflected signal; and abias resistor connected between a bias voltage and the diode bridge,wherein a second magnitude of the bias voltage is greater than themaximum magnitude and less than the first magnitude, and wherein thediode bridge filters voltages of the composite signal that are greaterthan the bias voltage and passes voltages of the composite signal thatare less than the bias voltage.
 20. The transceiver of claim 19 whereinthe diode bridge is a full-bridge.